The present invention relates to a method of forming inorganic pigments. More particularly, the present invention relates to a method of forming inorganic, pigments using one or more metal alloys as components of the rawbatch.
Inorganic pigments are typically produced by subjecting mixtures of selected metal oxides to high temperature calcination. During calcination, the mixtures of selected metal oxides undergo solid-state reactions and form solid solutions or crystalline structures consisting almost exclusively of crystals containing metal cations and oxygen ions. Inorganic pigments are used in a wide variety of applications, including the coloration of glass matrixes, ceramic bodies, polymers, and paints.
One well-known example of a inorganic pigment is cobalt-chromium-iron-manganese-nickel (Co, Fe, Mn, Ni)(Fe, Cr, Mn)2O4 black spinel. This inorganic pigment is typically formed by the high temperature calcination of a mixture of iron oxide (Fe2O3) as crystalline hematite and chrome oxide (Cr2O3), cobalt oxide (CoO), nickel oxide (NiO), and/or manganese oxide (Mn2O3). The raw material cost of certain metal oxide components such as chrome oxide, for example, can be a significant contributing factor in the final cost of the inorganic pigment.
The present invention provides a method of forming inorganic pigments using one or more metal alloys. Generally speaking, metal alloys can provide a less expensive source of some metal oxides that are incorporated as cations into the crystal lattice structure of the inorganic pigments. In accordance with the method of the invention, one or more metal alloys is preferably milled to a mean particle size of less than about 10 microns, mixed with other metal oxides, and then the mixture is calcined in the presence of oxygen in a rotary kiln to form the inorganic pigment.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.
Quite a number of metal alloys are readily available in the market. These metal alloys, which comprise alloys of two or more metals, are typically available as coarse powders or flakes. The term xe2x80x9ccoarsexe2x80x9d in this context should be understood as meaning that the mean particle size of the metal alloys is typically greater than about 20 microns. To facilitate their use in the present invention, it is preferable for such metal alloys to have a mean particle size of less than about 10 microns, and more preferably a mean particle size of less than about 5 microns. The mean particle size of the metal alloys can, if necessary, be reduced by conventional means, such as ball milling or jet-milling.
In order to produce an inorganic pigment in accordance with the invention, it is necessary to mix a sufficient amount of one or more metal alloys with a sufficient amount of one or more host metal oxides to form a rawbatch that, after calcination, will provide an inorganic pigment having the desired composition. The metal oxides used in the invention can comprise synthetic oxides, natural oxides, regenerated oxides, and other suitable metal oxides. In order to determine the relative amounts of metal alloys and metal oxides to use, it is necessary to ascertain the chemical composition of such materials.
Once the rawbatch is formed, it must be calcined. It is critical that the rawbatch be calcined in the presence of oxygen because the metal alloys must undergo oxidation during calcination. Oxygen can be supplied by the use of one or more oxidants in the rawbatch or, more preferably, by calcining the rawbatch in an oxidizing atmosphere such as ambient air. To promote contact between oxygen and the metal alloys during calcination, it is preferably to use a rotary kiln. Calcination is carried out at conventional temperatures, typically between 800xc2x0 C. and 1200xc2x0 C.
A wide variety of inorganic pigments can be produced in accordance with the method of the invention. One or more inorganic oxides can be mixed with appropriate amounts of one or more metal alloys and calcined to form inorganic pigments. Inorganic pigments formed in accordance with the method of the invention are essentially indistinguishable from inorganic pigments formed according to known methods.